The present invention relates to a method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a 4-wheel drive motor vehicle.
From the state of the art for all-wheel drive motor vehicles, it is known that, in one type of construction of all-wheel drive vehicles, one axle is permanently driven and the respective second axle is essentially hung onto this drive train, or is constructed as a so-called “hang-on” system. In a known application case, the rear axle is permanently driven and the front axle is connected as the hang-on system. For this purpose, the center differential in a drive train of the vehicle is replaced by a shiftable clutch by which the front axle can, here, be coupled to the rear axle.
In the control of the shiftably constructed clutch by the use of a control unit logic according to the state of the art, deviations of the tire periphery between the axles are not taken into account. If tire periphery deviations exist between the front axle and the rear axle, in the case of an all-wheel operation or in the case of a rigid coupling of the axle, the entire drive train will be distorted. The components, such as drive shafts, propeller shafts and differentials, will be subjected to a higher stress. Correspondingly, all participating components have to be designed for greater loads than would be the case within the scope of limit loads of a corresponding vehicle without the additional loads caused by tire periphery deviations. Furthermore, the tires will slip, which results in an increased abrasion and wear on their running surfaces.
An analogous situation occurs in the case of systems with a rear axle as a hang-on system. The relationships for insufficiently large diameters of the wheels at the front axle of a front axle hang-on system illustrated in the following with reference to an embodiment of the invention will then occur in the case of a rear axle hang-on arrangement when the rear axle is too small and vice versa. On the basis of these simple relationships between the two different hang-on systems, within the scope of the present disclosure, reference will only be made without any exclusion to a front-axle hang-on system.
In particular, a significant disadvantage of the prior art is that a distortion of the drive train with the exemplary disadvantages described above is accepted in a condoned manner by the construction of a rigid all-wheel drive. As an alternative, for example, according to the teaching of German Patent documents DE 37 21 626 C2 and DE 197 06 720 A1, the shiftable clutch is simply opened up to release a severe distortion in the drive train. Although in this manner any built-up distortion is compensated, the vehicle will then no longer have any all-wheel characteristics at least for the duration of this compensating operation. That means that, suddenly and in a manner that can almost not be predicted by the driver, in this condition, the vehicle acts only as a rear-wheel drive vehicle or as a front-wheel drive motor vehicle. Depending on the existing operating situation, this abruptly starting operation may have such a negative effect on the handling of a vehicle that, as a result, the vehicle may even become unstable.
It is an aspect of the present invention to create a method of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a four-wheel drive motor vehicle, and to create a corresponding system with an improved availability of a full four-wheel drive characteristic with a corresponding reliability of the driving dynamics of the corresponding overall vehicle.
According to the invention, this aspect is achieved by providing systems and methods of controlling a shiftable clutch in a drive train between a front axle and a rear axle of a four-wheel drive motor vehicle. One axle is driven directly and the other axle, as a hang-on system, is coupled by way of the clutch. The clutch is continuously acted upon by torque in an adjustable manner by way of a pilot control measure. Advantageous further developments of the invention are described and claimed herein.
A method according to the invention is therefore characterized in that the clutch is always acted upon by a torque in an adjustable manner by way of a pilot control measure. With respect to its extent, this torque is adjustable on the shiftable clutch itself or, for example, on a controlling hydraulic device.
This invention is based on the recognition that, even in the case of wheel diameter deviations and resulting distortions in a drive train of an all-wheel-driven vehicle, a complete torque compensation is not required. Within the scope of a tire tolerance logic, a torque is therefore defined by which the drive train may in each case be distorted. In order to be able to display this torque, the clutch is appropriately controlled in the course of the tire tolerance logic. Thus, the clutch remains essentially always closed during the driving operation. Therefore, as a further development of the invention, while the advantageous characteristics of an all-wheel drive are fully utilized, the desired torque distribution within the drive train onto a front axle and a rear axle can always be implemented. In the normal usage during the drive, an opening of the clutch, particularly for the purpose of a torque compensation within a distorted drive train, is avoided. Correspondingly, a motor vehicle operated according to a method of the invention, in contrast to known vehicles, can be operated by the permanent influence of the shiftable clutch while clearly limiting the slip and other negative phenomena by means of an essentially permanently acting all-wheel drive and with a high vehicle stability.
If the front axle is too small, the latter runs in a drive train of the prior art according to a concept with a rigid coupling of the axles at small engine torques in a negative torque, as illustrated in the following as a thin solid line VA in FIG. 2 of the drawing. Analogously, if the rear axle is too small, a negative torque should be expected there, as indicated as a thin broken line ha in FIG. 3. The driving torque distribution becomes dependent on tire deviations and the engine torque, but more precisely on a respective Cardan torque, which considerably impairs the driving dynamics. An analogous situation exist when the rear axle is too small. In an appropriate suitable neutral driving condition, for example, in the case of an unbraked, unpowered drive without curves and without an intervention of a driving dynamics control or of a DCS control, by means of the wheel speeds or the rotational speed deviation, the tire deviations between the front and the rear axle are determined. In this embodiment, the clutch is therefore opened only for a short time for this purpose. In the form of the average wheel inequality between the axles, the system then learns the respective conditions to which an adapted control concept is applied with the goal of letting the hang-on system operate as a permanent all-wheel drive without any slip.
In an embodiment of the invention, the defining of the clutch torque as the pilot control takes place mainly by the accelerator pedal position. It is formed such that the vehicle moves close to the fully locked range; that is, in a good approximation, it exhibits the behavior of a rigid all-wheel drive. If wheel inequalities exist in this case, a distortion torque will occur, which is superimposed on the ideal course of the torques at the two axles, so that, without the intervention by a method according to the invention, cumulative courses will occur which partially have very disadvantageous effects. The tire tolerance logic intervenes here in that the pilot control is correspondingly reduced. What should be taken into account here is, on the one hand, the type of distortion condition and/or which axle has a smaller rolling circumference, and, on the other hand, the load condition which is a result of the preceding sign of the effective Cardan torque as a trailing or the driving node. If the front axle is too small in comparison to the rear axle, in the case of the drive mode, when the Cardan torque rises from zero, the pilot control is restricted to a tolerated distortion torque by the tire tolerance logic. For Cardan torques smaller than zero, that is, in the trailing case, the tire tolerance logic restricts the pilot control to a fraction of the Cardan torque. Thus, despite an existing tire circumference deviation, a desired optimal distribution of the trailing torque to the axles can be achieved. If the rear axle is too small, point-symmetrical reflected torque curves occur so that the control tasks only have to be exchanged between the drive and the trailing case. Corresponding to these situations of driving dynamics, the torque acting upon the clutch is defined under the influence of tire tolerance logic, as will be explained in detail in the following with reference to diagrams of a concrete embodiment of the invention with an advantageously continuous course of the curve.
In an advantageous further development of the invention, however, the tire tolerance logic actively intervenes only when the deviation between the front and rear axle leaves a dead or intervention-free zone which, according to a further development of the invention, is selected as a function of the speed.
In this case, the system synchronizes the axle speeds in that it couples the front axle and the rear axle with one another by way of the transfer case such that at least a joint average speed of the axles is set also in the case of tire deviations. According to the above-described characteristics in embodiments of the present invention, in the control of the clutch in the form of a control unit logic, tire circumference deviations between the axles corresponding to a tire tolerance logic are taken into account by the determination of a tolerable differential torque in the controllable clutch. In an embodiment of the invention, a measurement of the rotational speeds of the front and rear axle, in the neutral straight run and while the clutch is opened for a short time, is carried out as described above. In contrast to methods of the prior art, it is thereby ensured that the all-wheel characteristic is eliminated only outside a condition that could tend to lead to a form of instability of the vehicle handling or the like. In addition, this phase with a loss of the all-wheel characteristic lasts only for a very short time, and furthermore, after the wheel inequality has been learned in one travel segment, the measurement does not have to be repeated. On the basis of the average rotational speed differences between the front axle and the rear axle, an occurring distortion torque is defined in the event of an overly distorted or firmly closed clutch or a deviation of the wheel diameter.
A method of the above-described type is based on a consideration of the torque conditions occurring because of the determined wheel diameter deviations in the case of a normalized high coefficient of friction of the road base. In an alternative embodiment, a method according to the invention is further developed such that, in the case of this consideration, respective actual road and coefficient-of-friction conditions are taken into account.
According to the above-explained teaching, a distortion of the drive train as a result of tire circumference deviations between the front axle and the rear axle is limited or displaced by the tire tolerance logic in a meaningful range. However, the degree of a possible distortion depends on the tire circumference deviations and the respective actual coefficient-of-friction conditions. A tire tolerance logic of the former type improves the driving dynamics of an essentially permanently all-wheel-driven vehicle but does not adapt itself, among other things, to changing road conditions. The most serious case is always assumed here—thus summer tires on a base with a high coefficient of friction. This has a special effect on the determination of the characteristic Cardan torque, above which, when the front axle is too small, the pilot control is no longer limited in order to utilize the driving torque specifically at the front axle for the acceleration. In the case of low coefficients of friction, the limitation of the pilot control could be eliminated in the case of clearly lower Cardan torques because the drive train is not as distorted as at a high coefficient of friction.
The tire deviations of the individual wheels are required as the input quantities for the tire tolerance logic. In the case of the former measuring method, these are determined in suitable driving conditions during an unbraked and unpowered drive without cornering or a driving dynamics control intervention or the like by means of free wheel speeds while the clutch is open. This has the disadvantage that the clutch has to be opened for the determination. The opening of the clutch, in turn, has the direct result that in this condition the vehicle has a pure rear wheel drive. Furthermore, in the case of this method, the observability of the tire deviations in the normally endeavored all-wheel operation when the clutch is closed will be lost.
In order to completely eliminate these basic disadvantages, in the case of an alternative method under the same suitable driving conditions, which were indicated above as being unbraked, unpowered, as well as without a cornering influence and/or control intervention, rotational speed differences between the front axle and the rear axle are determined at special clutch torques. On the basis of two value pairs of this type, via extrapolation, average tire deviations between the front axle and the rear axle as well as a distortion torque, which occurs in the case of an overlocked condition of the clutch, can then be determined. The method will be described in detail in the following on the example of a 0.6% diameter deviation between the front axle and the rear axle with reference to a drawing.
In order to achieve a high precision when using this method, clutch torques used for the identification are selected to be of sizes which are as different as possible, so that they indicate a large support basis with very clear rotational speed differences between the front axle and the rear axle. On the other hand, there is no falling below a minimally required clutch torque and no exceeding of a distortion torque maximally tolerated in the drive train.
On the whole, the invention provides a possibility of representing a permanent all-wheel drive in all driving situations, thus particularly without openings of the shiftable clutch forced for the purpose of the compensation of a distortion condition. An opening of the clutch is also no longer required for the determination and/or monitoring of tire diameter deviations. In a preferred embodiment of the invention, the opening of the shiftable clutch can therefore be completely eliminated.
Thus, according to the invention, in the case of a vehicle with tire deviations between the front axle and the rear axle, a distortion of the drive train occurs which is limited and tolerated in any condition. As a result, its components, such as drive shafts, propeller shafts and differentials, are less stressed than in the case of a permanent all-wheel drive with a rigid axle coupling. This approach reduces the wear, prolongs the service life and lowers cost and weight because the dimensioning can take place correspondingly. A negative influence of an excessive distortion of the drive train on the steering and the vehicle handling, which is noticeable to the driver, is eliminated. Furthermore, the tire wear is minimized and simultaneously the lateral control potential of the vehicle is not reduced because no wheel slips are forced by an excessive distortion. In addition, as a result of this method, the drive distribution within the vehicle is influenced such that the driving dynamics are improved during the drive as well as during trailing.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.